The present invention generally is directed to circuit arrangements and/or methods for improving the short term loading capacity of a circuit. More specifically, the invention is directed to arrangements and/or methods for increasing the loading capacity of a hybrid circuit arrangement having a resistive coating applied on at least one side of a glass or ceramic plate-shape circuit carrier.
Electrical circuits employing integrated resistors, for example, those that are utilized as protective resistors in public telephone switching systems, usually are manufactured using thick film or thin film technology. Yet such circuits should be capable of handling, at least for a brief durations, high electrical power loadings, for example, as occurs particularly in conjunction with network failures.
Background information relating to such circuits is disclosed in U.S. Pat. No. 5,254,969, the disclosure of which is fully incorporated herein by reference. As described therein, it is essential that balancing resistors, and line cards, perform in certain ways demanded by telephone companies, at three specified or standard levels of adverse conditions. The first level is one where the line card will survive and continue to function properly despite certain conditions, one being lightning transients. The second level relates to a condition where the balancing resistors are continuously overheating, for example because the line card is overheating due to being improperly connected by a technician. Relative to the this second level, there must be a thermal cutoff action to discontinue flow of current before the line-card circuit board starts to burn. The third level is one where there is a sudden application of high voltage, for example when a power line drops on the telephone line. Relative to this third level, current flow must be substantially instantaneously discontinued or small-diameter wires in the telephone system may melt.
To address the foregoing, U.S. Pat. No. 5,254,969 describes flat film type resistors intentionally caused to thermal shock fracture in response to a predetermined high-voltage overload condition. Heat sink portions for the resistors are formed in the circuit board by etching during the manufacture of the board.
Film circuits having an Al.sub.2 O.sub.3 substrate approximately 1 mm thick with a resistive coating applied on both sides and which can be loaded with approximately 2 kw for 100 milliseconds before a substrate failure occurs are commercially available. Therefore, it is not only the relatively short loading duration that is of concern but also the degree to which a failed substrate fragments or fractures since this fragmentation can lead to potentially conductive splinters that can be disbursed in an uncontrollable and dangerous fashion.